I have the opportunity to buy a cracked S4 and a mint S4, both for 350.

Is that a good price?

How much could it run me to fix the cracked one myself?

It's worth it IF it's a cracked glass ONLY and the LCD still works and no pixels are damaged.
Here, an S4 sells for around $325 in mint condition. I do not recommend you fix your S4. The part (the glass only) is cheap but if the LCD gets busted or damaged, which is overly common, it is basically worth nothing and the parts are as expensive as a new S4 would cost.
Take it somewhere to be fixed so if they mess it up, they pay for those expensive parts themselves and it doesn't come out out of your pocket.
Then just sell it to put extra cash in hand.
People fix them around here, in Atlanta, for anywhere between $80-$125.

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